1. Field of the Invention
This invention relates to an electrophotographic apparatus having a pre-transfer corona discharger provided between a developing device and a transfer charger to prevent fog toner adhering to the image light portion of an electrophotographic photosensitive member from being transferred to transfer medium. More particularly, it relates to an electrophotographic apparatus in which the charge polarity of toner adhering to the image dark portion is not varied but rather the amount of charge thereof is increased and moreover, only the charge polarity of toner adhering to the image light portion is varied to thereby prevent such fog toner from being transferred to transfer medium.
2. Description of the Prior Art
As an example of the prior art, a method and means for preventing transfer of fog toner by applying a very weak corona discharge of the opposite polarity to developing toner to a developed photosensitive member prior to image transfer are disclosed in U.S. Pat. No. 3,444,369. Briefly, the content of this patent is as follows.
For example, a selenium photosensitive member is used as the photosensitive member, and a dark portion latent image potential +800 V and a light portion latent image potential +200 V are formed on the surface of the photosensitive member by the Carlson method. When uniform corona discharge is effected on the photosensitive member having a latent image so formed thereon, by the use of a corotron, there occurs a difference in increased charge amount of corona discharge between the dark portion and the light portion. When the polarity of the corotron is .sym., in the dark portion of the photosensitive member, the charge amount imparted by the corotron leaks through the photosensitive member layer to the substrate of the photosensitive member as a charge amount greater than that allowed for the photosensitive member and the potential of the photosensitive member surface is maintained at +800 V. On the other hand, in the light portion of the photosensitive member, the surface potential is gradually increased by receiving .sym. charge from the corotron.
When compared in terms of the charge amount imparted to the photosensitive member surface per unit of time, in the dark portion of the photosensitive member, the electric field by its surface potential 800 V suppresses the .sym. corona discharge of the corotron more strongly than the electric field by the surface potential 200 V of the light portion of the photosensitive member, with a result that the charge amount imparted to the dark portion of the photosensitive member becomes smaller and the charge amount imparted to the light portion of the photosensitive member becomes greater. When there is toner adhering to the photosensitive member surface, the polarity of the toner adhering to the light portion is readier to vary. Therefore, the .crclbar. toner in the light portion readily changes to the .sym. polarity and therafter, when transfer corona of the .sym. polarity is applied to the back side of transfer medium, the toner in the dark portion is transferred to the transfer medium, whereas the toner in the light portion is not transferred to the transfer medium, thus providing a copy image free of fog.
Now, where the fog preventing means described in the aforementioned U.S. Pat. No. 3,444,369 is incorporated in a commercially available machine, irregularity is liable to occur to the transfer of the toner in the image dark portion and, when the environment in which the machine is installed has changed, there is a disadvantage that the ability to prevent the transfer of the toner adhering to the image light portion is reduced.
FIG. 1 of the accompanying drawings illustrates the reason therefor. Designated by 1 is a selenium photosensitive member. D denotes the image dark portion, and the surface potential of the photosensitive member is +800 V. Designated by 2 is .crclbar. toner adhering to the image dark portion, and the surface potential of the photosensitive member after the adherence of toner thereto is -700 V. L denotes the image light portion, and the surface potential of the photosensitive member is +200 V. Reference numeral 2 designates .crclbar. toner adhering to the image light portion, and reference numeral 3 denotes non-charged toner which electrostatically or mechanically adheres. In the light portion, the surface potential of the photosensitive member hardly varies after the adherence of toner thereto. A corotron is provided on the photosensitive member, and reference numeral 4 designates the discharge wire of the corotron. The spacing between the photosensitive member 1 and the corona wire 4 is 13 mm.
When, for example, a voltage of +6 KV is applied to the corotron, there occur the following electric fields between the toner surface and the corona wire:
In the dark portion: ##EQU1## In the light portion: ##EQU2## where E.sub.O represents the electric field by the corotron, E.sub.P represents the electric field by the photosensitive member, and E.sub.T represents the electric field by the toner.
The corona ions emitted from the corotron move through said electric fields and charge the adhering toner. As will be readily seen, the electric field E.sub.L in said light portion is substantially equal to the electric field E.sub.D in said dark portion. Therefore, according to the method of the prior art, it is very difficult to change the polarity of only the toner adhering to the light portion and not to change the polarity of the toner adhering to the dark portion. That is, part of the toner adhering to the dark portion is changed to the .sym. polarity, and then when transfer corona is applied, the toner changed to the .sym. polarity is not transferred, thus causing irregularity of the image.
When, for example, +4 KV is applied to the corotron, there occur the following electric fields between the toner surface and the corona wire:
In the dark portion: ##EQU3## In the light portion: ##EQU4##
The electric field in the direction of the dark portion is below the corona discharge starting electric field and therefore, the corona current generated in this direction is small. Thus, it is very rare that toner adheres to the dark portion by the corona generated by the corotron and the polarity of the toner is changed. On the other hand, .sym. corona is imparted to the toner adhering to the light portion, thereby enabling fog to be prevented.
However, since the intensity of the electric field E.sub.L is small, the movement speed of the corona moving from the discharge electrode to the photosensitive member is slow and this leads to a disadvantage that too much time is required to impart sufficient .sym. corona to prevent fog. Also, since the corotron is used in the vicinity of the corona starting voltage, the generation of corona is too much suppressed when temperature and humidity change, and this leads to a disadvantage that prevention of fog is not sufficiently effected.